The invention relates to three-phase, high-current switchgear apparatuses, with or without neutral, comprising pole compartments connected in parallel.
The document EP 0,320,412 describes a three-phase, high-current switchgear apparatus, in this case a circuit breaker, comprising two adjacent pole compartments per pole, and two adjacent pole compartments for the neutral. Each pole compartment comprises two separable contacts each connected to a contact strip. The pole compartments of the same phase are twinned by electrically connecting the contact strips two by two by means of a connecting strip. Each pair of twinned poles thus constitutes a current loop formed by the two connecting strips and the conductors of the two pole compartments. Each phase is connected to a busbar at the level of its connecting strips.
It so happens that when the circuit breaker is closed, in balanced three-phase AC operation, the mutual electromagnetic interaction between the phase currents gives rise to a non-homogenous distribution of the currents in the bars and in the conducting parts of the circuit breaker. The electromagnetic field generated by each of the conductors influences the current distribution in the other conductors. Globally, a non-homogeneous temperature rise of certain conducting parts is then observed, known by the name of proximity effect.
On account of the fact that the electromotive forces induced by flow of the current in the different branches of circuit increase with the circuit breaker rating, the heterogeneity is all the greater the higher the circuit breaker rating. For a rated phase current of 6300 A for example, a distribution of the rms value of the current of about ⅓, ⅔ can be observed between the two compartments of one and the same phase, so that the current intensities or temperatures reached at certain points may exceed the limits fixed by the standards.
To stabilize the current distribution between the two branches corresponding to two twinned poles of the same phase of a low-voltage power circuit breaker, it has already been proposed in the document FR 2,063,078 to make the conductors of the two branches cross, so as to superpose the two portions of conductors in which currents are flowing in opposite directions, and to incorporate a magnetic circuit entwining the two superposed conductors. From the indications given by this document, it is apparent that such a device compensates the differences of current intensity between the two branches of one and the same phase generated by the electrical resistance differences, for example at the level of the contact resistances of the contacts of each of the branches. Knowing that in practice the differences between contact resistances of two poles are about 5%, this device proves effective for small current intensity variations between the two compartments of the same phase. However, the device proves difficult to implement when the unbalance between phases becomes great or when the rating of the apparatus increases. In particular, crossing of the conductors in a single magnetic circuit, although it does not cause any problem for medium intensity currents of about 630 A, can no longer be applied for very high current apparatuses, in excess of 4000 A in particular, for obvious reasons of dimensions. And yet it is precisely on very high current apparatuses that the effect of the mutual induced electromotive forces between branches of the electrical circuit internal to the apparatus becomes critical. The teachings of the document FR 2,063,078 therefore do not enable a solution to be provided to the specific problem arising from the proximity effect between phases described previously.
Another method for stabilizing the current distribution between the two branches corresponding to two twinned poles of any one phase of a low-voltage power circuit breaker would consist in arranging the two poles of each phase in non-contiguous manner, for example so that the two poles of each phase are separated from one another by one of the poles of each of the other two phases. If we number the six pole compartments from one side of the circuit breaker to the other from 1 to 6, we would thus have: poles 1 and 4 for a first phase, poles 2 and 5 for a second phase, and poles 3 and 6 for the third phase. Such an arrangement does however give rise to large dimensions at the level of the busbars of the different phases and of the bridge connections between poles of the same phase. Moreover, it prevents any interaction device between pole compartments of the same phase: it makes it impossible in particular to provide a communicating orifice between the two pole compartments of the same phase, as described for example in the document FR 2,778,788, an orifice which enables an adequate distribution of the breaking energy to be ensured in case of opening of the apparatus on a fault.
The object of the invention is therefore to improve, or even optimize, distribution of the electrical current and temperatures between the twinned poles composing the phases of a three-phase switchgear apparatus with contiguous twinned poles, limiting the additional cost arising from the arrangements adopted as well as the increase of the dimensions of the apparatus.
According to the invention, this objective is achieved by means of a three-phase electrical switchgear apparatus comprising a case made of insulating material comprising at least six pole compartments arranged side by side, each phase comprising:
two adjacent poles, each pole comprising
one of said pole compartments and
a pair of separable contact means formed by a first and a second contact means;
a first bridge connection electrically connecting the first contact means of the two adjacent poles of said phase;
a second bridge connection electrically connecting the second contact means of the two adjacent poles of said phase;
one of the three phases constituting a center phase bounded on each side by the other two phases which each form a side phase, one of the two poles of each side phase forming an inner pole whose pole compartment is adjacent to one of the pole compartments of the center phase,
and wherein:
each of said inner pole compartments of the two side phases comprises a magnetic compensation circuit arranged between one of the two bridge connections of said phase and the pair of contact means of said inner pole compartment,
the other two pole compartments of the two side phases are not provided with magnetic compensation circuits.
Indeed, in balanced three-phase operation, the electromagnetic interaction between the phases located in the same plane has the effect of increasing the intensity of the current flowing in the inner poles of the side phases to the detriment of the current flowing in the outer poles of these same phases. It is therefore also the inner poles of the side phases which are the most affected by temperature increases by Joule effect. According to the invention, by arranging the magnetic circuits judiciously on the inner branches of the side phases, an impedance is introduced into the circuit which makes the current decrease in targeted manner in the pole compartment where the magnetic circuit is situated. The desired result is thus achieved with minimum additional cost.
The fact that the bridge connections form part of the apparatus enables the influence of the parts of the circuit situated outside the apparatus, in particular the influence of the supply busbar, to be eliminated. In other words, the current loops of each phase formed by the conductors of the two pole compartments and the source-side and load-side bridge connections are defined at the time the apparatus is designed and do not depend on the on- site assembly. It is therefore possible to calibrate the magnetic circuit judiciously so as to obtain the required compensation for given power supply conditions. The compensation obtained is then independent of the composition of arrangement of the source-side and load-side circuits and in particular of the arrangement of the busbars.
Advantageously, for each inner pole compartment, the magnetic compensation circuit forms part of a current transformer comprising in addition a secondary winding for supply of an electronic circuit of the apparatus. The switchgear apparatuses are often provided with at least one magnetic supply circuit arranged on each of the pole circuits. One of the existing magnetic supply circuits is then used for compensation, and the magnetic supply circuit of the adjacent pole compartment of the same phase is simply not fitted. The effect sought for is then achieved with a reduced cost with respect to the unit cost of a pole.
Advantageously, for each inner pole, the magnetic circuit comprises:
a main part surrounding a conducting part of one of the contact means, a portion of this main part constituting a core for the secondary winding; and
a magnetic shunt branch-connected on said portion constituting the core of the secondary winding, the magnetic shunt comprising a total or partial air-gap.
The air-gap is said to be partial when it is not zero over a part of the cross-section of the shunt and is zero over the remaining part of the cross-section. This type of circuit, described for example in the document EP 0,704,867, classically provides the advantage of shunting the core providing the power supply to the secondary circuit when the primary current exceeds a certain threshold value. This type of magnetic circuit here in addition enables the two functions of power supply and compensation performed by the magnetic circuit to be separated. The core designed for the function of supplying power to the electronic circuit and the shunt performing the function of compensation and of peak-clipping above the threshold value can in fact be dimensioned relatively independently from one another.
According to a preferred embodiment, for each inner pole compartment, the current transformer is situated inside said pole compartment. The location usually reserved for the supply current transformer is then used.
In other words it is possible with such an arrangement to adopt a common architecture for an apparatus with one pole per phase and for an apparatus with two twinned poles per phase.
According to another embodiment, for each inner pole compartment, the current transformer is situated outside said pole compartment. This arrangement provides more space for housing the magnetic circuit. It moreover prevents the temperature rise of the magnetic circuit caused by iron losses from causing a temperature increase of the corresponding inner pole compartment.
Preferably, the magnetic compensation circuit is dimensioned in such a way that, when the apparatus is supplied in balanced three-phase operating conditions at its rated voltage and has its rated current flowing through it at its rated frequency, each magnetic compensation circuit generates an impedance in the inner pole compartment such that the current flowing in the inner pole of each side phase is lower than or equal to the current flowing in the other pole of the same phase. The strict equality between the rms values of the currents flowing in the two branches of a side phase enables a balance to be obtained between the energies dissipated in the two pole compartments of any one phase. But it is known that in numerous configurations, heat removal is potentially greater for the outer poles of the side phases. In this case, an over-compensation enables most of the current to be switched to the compartment which is easiest to cool.
Advantageously, the bridge connections are fixedly secured to the case. The switchgear apparatus is then delivered to site with its bridge connections fitted. The bridge connections are preferably fixed outside the pole compartments.
According to a particular embodiment, the apparatus is a plug-in unit and comprises:
a frame in which the case is able to slide between a plugged-in position and a plugged-out position,
connection strips fixedly secured to the frame, each contact means having one of the connection strips corresponding thereto,
draw-in finger contacts, each of said contact means having one or more draw-in finger contacts corresponding thereto and providing a disconnectable electrical connection between said contact means and the corresponding connection strip,
said bridge connections being arranged in such a way that, for each phase, the first bridge connection electrically connects the first contact means via the draw-in finger contact or contacts corresponding to said connected first contact means and that, for each phase, the second bridge connection electrically connects the second contact means via the draw-in finger contact or contacts corresponding to said connected second contact means.
This arrangement enables the currents flowing in the connecting circuits between the connection strips and the contact means, including the draw-in finger contacts, to be taken into account in compensation.